1. Field of the Invention
The present invention relates generally to porous metal surfaces and methods for forming such surfaces, particularly for use in medical prostheses.
2. Brief Description of the Prior Art
It is well-known in the medical implant art to provide a porous surface on selected areas on a medical prosthesis to permit the bone cement, or ideally the bone itself, to penetrate the voids in the surface in order to establish and maintain a strong mechanical bond with the implant. A frequently employed technique for creating such an active implant surface area involves the selective placement of a porous coating on the implant device. The most commonly used porous coatings are gravity or pressure sintered spherical powders, diffusion bonded metal fibers and plasma sprayed powder coatings. Exemplary of such sintered metal powder coatings are those described in our U.S. Pat. Nos. 4,612,160 and 4,854,496.
Titanium and titanium alloys have experienced wide usage as medical implant materials, especially for medical prostheses such as orthopedic devices in the form of knee and hip joints. Diffusion bonded metal fiber coatings have been produced from titanium wire in the form of random porous fiber metal coatings. Likewise, in plasma sprayed coatings, it is also known to utilize either commercially pure titanium or titanium alloy powders. The desirability of producing porous surfaces on medical prosthetic devices is well-known as seen, for example, in U.S. Pat. No. 3,855,638 to Pilliar, U.S. Pat. No. 3,605,123 to Hahn, U.S. Pat. No. 4,017,911 to Kafesjian and U.S. Pat. No. 3,808,606 to Tronzo.
The various medical factors involved in bone or tissue ingrowth, including those involved in bone cement adhesion, are documented and well-known to those in the medical implant art. The medical community has also clearly indicated that control of the pore size in porous coatings is highly desirable and that dimensional control of the implant itself is absolutely necessary for satisfactory prosthetic devices. In addition, careful control of the metal chemistry and the elimination of sources of contamination are also recognized to be essential for implanted prosthetic devices. As noted previously, it has been common practice to produce porous surfaces by applying a porous coating to the implant substrate. It has been found in some instances, however, that after prolonged periods of use, portions of the porous coating may break loose from the coating mass. When such an event occurs, the loose porous coating material becomes a contaminant in the surrounding tissue. Naturally, such contamination is highly objectionable since it may require surgical intervention in order to correct the problem.
It is certainly desirable to prevent such contamination while still providing sufficient porosity in the implant surface to permit proper mechanical interlocking by cement adhesion and bone or tissue ingrowth. The present invention solves these prior shortcomings by providing an improved porous surface which eliminates the opportunity for surface breakaway and subsequent tissue contamination. The present invention provides a process for forming a porous surface on or in a workpiece such as a medical implant, having closely controlled porosity which may be uniformly dispersed or varied in spacing and in size in pre-selected areas over the implant surface. The process of the present invention further provides a porous surface which is more economical to produce than comparable processes such as the commonly used powder metallurgy sintering or diffusion bonding processes of the prior art. The present invention also provides a medical implant device having extremely close dimensional tolerances with closely controlled porous surface areas having pores or cavities of selected size and spacing. The present invention provides a porous surface and process for producing same in which the size of each cavity with respect to its diameter and depth may be closely controlled in either a constant or varying pattern across the workpiece to provide consistent high quality surfaces. In addition, the cosmetic appearance of the porous surface is enhanced.